1. Field of the Invention
The present invention relates to a method of manufacturing a monolithic ceramic electronic part, and a monolithic ceramic electronic part. Particularly, the present invention relates to an improvement for making uniform a sintering degree of a laminate provided to a monolithic ceramic electronic part.
2. Description of the Related Art
An example of monolithic ceramic electronic parts concerned in the present invention is a monolithic ceramic capacitor. FIG. 1 is a sectional view showing a laminate 1 provided to a monolithic ceramic capacitor and is taken along line III—III in FIG. 2.
The laminate 1 comprises a plurality of laminated ceramic layers 2, and a plurality of internal electrodes 3 serving as internal conductor films extending along a plurality of interfaces between the respective ceramic layers 2 positioned in an intermediate portion in the lamination direction.
The laminate 1 is produced as follows.
First, a plurality of ceramic green sheets each containing a ceramic powder are prepared for forming the ceramic layers 2 by burning, and internal electrodes 3 are respectively formed on the ceramic green sheets used for the ceramic layers 2 positioned in the intermediate portion. The internal electrodes are respectively formed by, for example, printing conductive paste containing a conductive metal component on the ceramic green sheets.
Next, the plurality of ceramic green sheets are laminated to obtain a green laminate. In order to prepare the green laminate, ceramic green sheets for external layers, on each of which an internal electrode is not formed, are laminated on both lamination-direction ends of the laminate of the ceramic green sheets on which the internal electrodes 3 were respectively formed.
Next, the green laminate is burned to obtain the laminate 1 shown in FIG. 3.
As shown in FIG. 2, external electrodes 4 are formed on both ends of the laminate 1 to complete a desired monolithic ceramic capacitor 5. The ceramic green sheets for the ceramic layers 2 positioned between the respective internal electrodes 3 generally have the same composition as the ceramic green sheets used for the external layers. On the other hand, it is known that the conductive metal component contained in the internal electrodes 3 is partially oxidized, and diffuses and dissolves into the ceramic green sheets between the respective internal electrodes 3 and into the ceramic green sheets adjacent to the internal electrodes 3 during the step of burning the green laminate. Particularly, as the thickness of the ceramic layers 2 decreases, the oxide of the conductive metal component contained in the internal electrodes 3 readily diffuses to increase the oxide concentration in each of the portions between the respective internal electrodes 3.
The concentration of the oxide of the conductive metal component influences the characteristics of the monolithic ceramic capacitor after burning. In order to decrease the influence, the composition of the ceramic powder contained in the ceramic green sheets is appropriately controlled.
The distribution of the internal electrodes 3 in the laminate 1 is such that the internal electrodes 3 are formed in portions except at both ends 6 of the laminate 1 in the width direction and both ends of the laminate 1 in the lamination direction, i.e., in the external layer portions 7. Therefore, diffusion and dissolution of the conductive metal component contained in the internal electrodes 3 do not reach to both ends 6 of the laminate 1 in the width direction and the external layer portions 7, and thus the composition in these portions is different from the portions near the internal electrodes 3.
As a result, an insufficient sintering region 8, in which crystal grains are not grown, is formed at both ends 6 of the laminate 1 in the width direction and the external layer portions 7, as shown by oblique lines in FIG. 1.
It is more difficult to prevent the insufficient sintering region 8 from being formed in the external layer portions 7 than at both ends of the laminate 1 in the width direction. This is because the conductive metal component contained in the internal electrodes 3 can be sufficiently diffused to both ends 6 of the laminate 1 in the width direction by increasing the width of the internal electrodes 3, i.e., narrowing a margin where the internal electrodes 3 are not formed. However, the internal electrodes 3 cannot be formed in the external layer portions 7 at all.
In the insufficient sintering region 8, a sintering shrinkage does not sufficiently proceed, and thus stress occurs in the direction in which the laminate 1 is extended vertically as shown by arrows 9 in FIG. 1, or in the direction in which the laminate 1 is extended laterally.
Particularly, when vertical stress is applied to the laminate 1, slight peeling occurs between the internal electrodes 3 and the ceramic layers 2, thereby causing a defect in the insulating resistance of the monolithic ceramic capacitor 5 in some cases. In the extreme case, a larger amount of peeling, i.e., delamination, occurs.
Furthermore, moisture can enter the insufficient sintering region 8. Therefore, moisture enters the insufficient sintering region 8 during wet-polishing the laminate 1 after burning. In the next step of applying conductive paste containing, for example, copper, and baking the coating to form the external electrodes 4, therefore, the volume of the moisture entering the insufficient sintering region 8 becomes increased by the heating and acts to blow away a part of the laminate 1 or a part of the external electrodes 4. Consequently, a chip 10 occurs as shown in FIG. 2.
FIG. 3 is an enlarged sectional view showing the chipped portion 10 shown in FIG. 2. In FIG. 3, a form without the chip 10 is shown by dotted lines. As can be seen from a comparison between the form shown by the dotted line and the form shown by a solid line in FIG. 3, the chip 10 occurs in both a part of the laminate 10 and a part of the external electrodes 4. The chip 10 results in a failure of the appearance of the obtained monolithic ceramic capacitor 5.
Furthermore, beyond the moisture encountered in wet polishing, a plating solution used in wet plating and atmospheric moisture can possibly enter the insufficient sintering region 8. When the moisture, the plating solution or atmospheric moisture enters and accumulates in the insufficient sintering region 8, not only chip 10 occurs but also peeling between the internal electrodes 3 and the ceramic layers 2 occurs.
When sintering conditions are made severer by, for example, increasing the sintering temperature in order to sufficiently achieve sintering in the insufficient sintering region 8, the regions between the respective internal electrodes 3 are excessively sintered to increase the dielectric loss of the obtained monolithic ceramic capacitor 5 or cause the temperature characteristic of capacitance to deviate from a desired range, thereby causing an abnormality in electric characteristics.
Although problems with a monolithic ceramic capacitor are described above, the same problems possibly occur in monolithic ceramic electronic parts other than the monolithic ceramic capacitor.